A silver paste is known as a wiring material and a bonding material for electronic components. The silver paste is a paste obtained by dispersing conductive silver particles in an organic solvent. However, silver tends to cause an ion migration phenomenon. The ion migration phenomenon is the phenomenon that describes a short circuit in a wire of a metal on insulation that is caused by some of the metal migrating on the insulation. Also, silver is expensive and therefore unfit for industrial use.
A wiring material and a bonding material that can be substitutes for the silver paste have therefore been contemplated. For example, use of a copper paste has been contemplated. Copper particles included in the copper paste have electrical resistivity comparable to that of silver particles and tend not to cause the ion migration phenomenon compared to silver particles. Examples of methods for producing copper particles include a mechanical pulverization method and a vacuum method. The mechanical pulverization method involves pulverizing a mass of copper using a pulverizer. The vacuum method is for example a method such as disclosed in Patent Literature 1. More specifically, according to the vacuum method, copper is heated and evaporated in a reduced pressure environment. Subsequently, the resulting copper vapor is cooled thereby to condense and solidify the copper. Through the above, a colloid of copper particles is produced.